(1) Field of the Invention
The present invention relates to a coating composition which provides a coating film to be formed on a work article surface, exhibiting a varistor property with respect to electrical characteristics and to a process for production of a coated metal article having said composition applied on a metal article and exhibiting high property of electrodeposition coating, corrosion resistance, forming property, and surface treatment property.
(2) Description of the Prior Art
Hitherto, with respect to the coating of general industrial articles, especially automobile body components, cationic or anionic electrodeposition coating has been widely used because of advantages thereof such as superior uniformity in thickness of the resulting coating film, throwing property, and corrosion resistance. However, even the electrodeposition coating is not always almightly. For example, in electrodeposition coating of non-treated steel plates, aluminum articles, zinc-plated articles, zinc alloy-plated steel plates, etc., because of high activity of a work article surface, there are caused disadvantages such as surface roughening and formation of pinholes on a coating film to be formed and, therefore, it has been required to undergo any pretreament or precoating.
In particular, in order to meet a requirement for improving the corrosion resistance which is highly advanced from year to year, there have been made various designs regarding steel plates for automobile body.
For example, a precoated steel plate known as a name of zincrometal, comprising a stell plate on which a zinc-rich paint with corrosion resistance has been previously applied; a zinc alloy-plated steel plate comprising a steel plate plated with zinc or a zinc alloy comprising zinc and one or two or more of Al, Fe, Mo, Co, Cr, etc.; and a composite coated steel plate prepared by subjecting the foregoing zinc alloy-plated steel plate to a chromate treatment and then applying thereon an organic coating film containing colloidal silica in a thickness of about 1 micron are put into practical use.
In painting automobiles, these coated steel plates are formed, assembled into body shapes or component articles, subjected to cationic or anionic electrodeposition coating, and then completed by intermediate coating and top coating for bodies or by rust-prevention coating for component articles, respectively. In this case, the zincrometal involved such a defect that because of zinc dust to be contained in a large quantity, a coating film to be formed is brittle so that a problem arises at the time of forming, while the zinc alloy-plated steel plate involved such a defect that because of its surface activity, a coating film to be formed likely causes surface roughening and formation of pinholes. Furthermore, the composite coated steel plate which has been developed in order to overcome these defects and achieve high corrosion resistance involved such a defect that since a coating film to be formed is electrically insulative, in order to obtain a good property of electrodeposition coating in the electrodeposition coating to be subsequently carried out, the coating film must have a film thickness of 1.+-.0.3 microns, whereby a large number of management steps are required for the production so that even a slight dispersion variation of the film thickness results in deterioration of the property of electrodeposition coating. In any of these cases, it is the present status that an immediate improvement is demanded.
There have been made investigations based on an assumption that the above-described property of electrodeposition coating could be improved by imparting electrical conductivity to coating films to be formed in these coated steel plates involving such defects. As a result, it has become revealed that in the case of applying a coating composition to which electrical conductivity has been imparted on a steel plate, since electrical conductivity is imparted to a coating film to be formed therefrom, the subsequent property of electrodeposition coating is improved, whereby even when a some dispersion variation in the film thickness of the coating film is present, it is possible to undergo electrodeposition coating, resulting in formation of a good electrodeposition coating film.
However, the above-described coating film formed on such a steel plate always exhibits electrical conductivity. That is, the aforesaid coating film exhibits electrical conductivity not only at the time of electrodeposition coating but also after the formation of an electrodeposition coating film. For this reason, it has become clear that since a slight amount of a current readily flows even at such a low voltage as the corrosion potential to promote the corrosion, a highly corrosion-resistant coating film required in, e.g., automobile bodies, cannot be obtained.